"Michael A. Terrell" ) writes:


There are Murata (and clone) 455 KHz ceramic filters in old cordless
phones and pagers as well. i always ttake the junk cordless phones
people offer and I am trying to get the scrap circuit boards from a
local pager and cell bhone rebuilding company.

But those will be quite wide, given that they are for narrowband
FM.

The suggestion of CB sets is good, since those should tend to be
narrower than AM broadcast receiver filters. The sad part is
that the digitally tuned Delco car radios that I use as
everyday radios use a 450KHz IF (presumably because of the math
for the synthesizer), so one can't simply move a CB filter to
the car radio.

Sadly, it's easier to scrounge up wider filters than narrower
ones. Old clunky cellphones are also a good source of "roofing
filters" and 455KHz filters, but they too will be wider than
desired for AM reception. You're much more likely to come across
something that uses FM than AM or even less likelier SSB. By
the time you find something that has an SSB filter in it, you
have to think carefully about whether it's more valuable
as an intact piece of equipment. I've found CB sets in the garbage,
but never an SSB set (and never junked shortwave receivers, either.

One line of pursuit might be cascading wider filters, at least of
the bandwidth found in the average AM broadcast receiver. People have
talked about doing this, though I've not seen much detail. Can you
get much variation in bandwidth by fiddling with the termination (and
without adding bad ripple to the passband)? Use small coupling capacitors
to link a few of the filters together, getting something narrower
than just a few cascaded filters (which generally just improves the skirts).

Michael